Relationships between bilateral asymmetry and tonic immobility reaction or heterophil to lymphocyte ratio in five breeds of chickens.

Bilateral asymmetry of several morphological traits, tonic immobility reaction, and leucocyte ratio were studied in hens and cocks from four Spanish breeds of chickens (Castellana, Buff Prat, Red Villafranquina, and Barred Red Vasca) and an F2 cross between Castellana and Buff Prat (C x BP - F2). Any two or all three types of bilateral asymmetry (fluctuating asymmetry, directional asymmetry, and antisymmetry) occurred together for the same trait in the different breeds. There was a significant difference in relative asymmetry among breeds (P < 0.05) for feather and spur lengths in males, and considerable negative heterosis for relative asymmetry of leg and wing lengths in C x BP - F2 males (-38 and -12%, respectively). The degree of relative asymmetry varied among traits and was between 3 and 12 times greater for spur length in males and for wing bay area in Villafranquina males. There were significant differences among breeds (P < 0.05) for tonic immobility duration. The duration of tonic immobility was positively associated with the mean relative asymmetry of all traits in females from the Villafranquina breed (r = 0.32; P < 0.05), but this finding was not consistent across the different breeds. No significant difference was found among breeds in terms of heterophil to lymphocyte ratio. Although there was no significant association between mean relative asymmetry of all traits and heterophil to lymphocyte ratio, males from the Villafranquina breed had a positive and significant relationship between relative asymmetry for wing bay area and leucocyte ratio (r = 0.34; P < 0.05). These results suggest that relative asymmetry measures did not provide a general tool to assess fear or stress susceptibility.